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When I look up at the stars I'm always astonished and filled with wonder at how large the Universe really is. It looks like I could reach up and pluck the stars out of the sky like gems set in a velvet dome. A magical image! But the reality is even more compelling to me. Stars are immensely brilliant glowing balls of superheated plasma floating in space incredibly far apart.

But how do we know this? We can't use a ruler to measure the distance to stars, and all we usually see of stars are points of light. Even using our most powerful telescopes and most sophisticated techniques there are only a handful of stars that we can “resolve” or see any details on. How we know what stars are without being able to resolve them is a long and complicated story starting with rainbows and a man named Fraunhofer. But how we know how far away stars are is something more approachable in this blog.

Illustration of a stellar parallax.

The most powerful technique astronomers use is that of parallax. It is actually similar to how our brain takes the images from our two separate eyes to judge distance. Try it yourself: hold a finger up in front of your face. Now look though each eye separately without moving your head or finger. You'll see your finger appear to jump from spot to spot against the background objects. The farther away your finger is the smaller the apparent jump. If you could increase the separation between your eyes the jump would appear larger. By measuring the size of this apparent jump you can measure the distance to objects. Your brain does this automatically to give you a three-dimensional sense of the world.

Now the stars are much much much farther way than anything in our usual experience so the distance between our two views (the “baseline”) has to be much larger. Astronomers are used to travelling to distant place in the world, so this wouldnít seem to be a problem… But it turns out that even opposite sides of the world isn't enough! Luckily all of us routinely travel much much further every year… from one side of the Sun to the other in half a year as the Earth goes through the seasons. But the distances to the stars is so large that even with this tremendous 186 million mile baseline the apparent parallax jump is still very very tiny. Even for the closest star the jump is less than a thousandth of a degree. From there out it just gets smaller. So getting parallax distances is a big deal, especially to the more distant stars.

Late last year the European Space Agency launched a mission that I am very excited about: GAIA. GAIA is a satellite designed to map the heavens to unmatched precision. It will measure the positions of a billion stars across the whole sky. Because it will do this many times over a period of years it will see the tiny wobbles from parallax as the Earth goes around the Sun. This will allow us to measure the distance to a billion stars across our Galaxy. Not just a few thousand, or even a million... a billion! For the first time ever we will have an accurate map of a large part of the Milky Way Galaxy.

Not only will we know how far away the stars are, but we'll know their true brightness also. And how quickly they move through space. Until now we've only had this information about a relative few stars. With this wealth of information astronomers be able to figure out not only where the stars are in three dimensions, but also how they will dance around and by each other for millions of years into the future.

But the benefits don't stop there! So much of astronomy is based on knowing accurate distances to stars that practically every area of astrophysics will be affected. In some topics the results of GAIA will be simply welcome confirmation, but in others it will challenge old ideas and perhaps lead to entirely new approaches. And we'll have a massive database to search through and find amazing new discoveries. Who knows what we'll uncover!

Written by Geza Gyuk, vice president for astronomy, research, and collections at the Adler Planetarium.